1. Field of the Invention
The field of the invention is that of the acquisition of seismic data. More precisely, the invention relates to equipment for analysing sea beds. The invention in particular relates to the industry of oil prospection via the seismic method, but can be applied to any field implementing a seismic data acquisition network in a marine environment.
2. Background of the Invention
The operations of acquiring, in the field, geophysical data, conventionally implement networks of sensors. These sensors are most often hydrophones; however said operations can also implement geophones, accelerometers or any other type of sensors. The hydrophones are distributed along cables in order to form linear acoustic antennas commonly designated by the terms “streamers” or “seismic streamers”. The network of seismic streamers is drawn by a seismic boat.
A seismic streamer is comprised of an assembly of individual sections instrumented with seismic sensors and associated digital-analogue conversion electronics. The seismic boat draws also one or several seismic sources comprised of an air gun network, water guns or acoustic vibrators. The pressure wave generated by the seismic source crosses the head of water and insonifies the upper layers of the sea bed. A portion of the signal is refracted by the interfaces and the inhomogeneities of the oceanic crust. The resulting acoustic signals are then detected by the seismic sensors distributed across the entire length of the seismic streamers. These acoustic signals are conditioned, digitised and retransmitted by the telemetry of the seismic streamers to the operator station located on the seismic boat where the raw data processing is carried out.
Seismic exploration campaigns can be scheduled over periods extending over several months. In addition, even in the case of unfavourable weather conditions, it is avoided, as much as possible, to return the streamers to the vessel, as the operations of returning/deploying are in practice particularly tedious and long. Indeed, each streamer has a length of several kilometers (up to 12 kilometers). This results in that seismic streamers can remain immersed in sea water for several consecutive months (with period of immersion of 6 to 12 months being frequent). Moreover, seismic streamers are generally immersed at a shallow depth (a few meters), and are generally dragged at low speed (less than or equal to 5 knots). Therefore, the tubular structure of the streamers is subject to fouling, in particular due to the proliferation, on the external layer of the seismic streamers, of microorganisms or bio-fouling.
However, these bio-fouling generate in time several disadvantages, among which:                they generate hydrodynamic flow noise: the proliferation of the bio-fouling can result in the formation on the external layer of the seismic streamers of amalgams or incrustations likely to disturb the measurements to be carried out;        they tend to increase the drag of seismic streamers and, consequently, the consumption in fuel of the boat that is tugging them;        during the winding of seismic streamers on the winches of the tug boat, the bio-fouling is deposited and fouls the equipment, and generate, after a few days, a stench due to their degradation in the air;        certain types of bio-fouling have a shell that can pierce the sheaths of the seismic streamers, which can give rise to water intake inside the seismic streamer, or to an oil leak (in the case of a seismic streamer incorporating a filling fluid, such as kerosene).        
Several techniques have been proposed in prior art in order to overcome the disadvantages generated by the proliferation of bio-fouling on the external sheath of the seismic streamers.
A technique is in particular known disclosed by patent FR-2 847 987, according to which it is proposed a cleaning device of the external sheath of seismic streamers, which includes:                a tool for treating the seismic streamer, comprising rotary brushes, as well as blades to abrade the incrustations formed on the external sheath of the streamer;        means for positioning and guiding the device along the streamer.        
The structure of such a device is relatively complex, in the sense where the design of the latter can be provided in order to allow for its passage on protruding elements on the tubular structure of the seismic streamers, these elements able to be comprised of floaters, or of trajectory correction devices (commonly designated by the term “bird”). Such devices have been experimented with, and then abandoned in practice, due to their cost and especially due to the fact that, most of the time, become separated from the seismic streamers and become lost in the sea beds.
“Anti-fouling” paints are also known, conventionally used on boat hulls. However, “anti-fouling” paints generally contain TBT (tributyltin) which is now well known for its toxicity. Such paints are therefore unacceptable from an environmental standpoint (pollution of sea water) as well as in relation to the safety of persons likely to be in contact with TBT paint during the manipulation of seismic streamers. Furthermore, the techniques for applying “anti-fouling” paint are hardly compatible with the technical and economical constraints linked with seismic streamers. Indeed, the techniques for applying these paints imply an operation of projecting paint onto the support. This operation of projection can be carried out by operators, using a projection gun. In this case, the projection operation is particularly long and costly due to the high number of kilometers of streamers to be treated. In addition, it is necessary to comply with a drying time for the paint, which implies storing the seismic streamers in positions that take up a considerable amount of space in order to avoid any contact of the portions of treated streamers with themselves (which excludes the winding of seismic streamers); such storage is consequently not viable economically.
It has also been proposed to carry out the projection of “anti-fouling” paint using projection rings, inside of which the tubular structure of the seismic streamers circulates, this with a paint having the capacity of drying dry in contact with water. The principle in implementing this technique consists in mounting the projection rings on the streamer tugging vessel and in carrying out the operation of treatment using rings in the open sea, the seismic streamers being stored in the water as they are treated. Such a technique thus makes it possible to overcome the problems of storage of seismic streamers during the drying phase of the paint, but implies very high implementation costs. Indeed, the equipment costs for boats are substantial in the sense where it is necessary to provide as many projection rings as there are winches for winding seismic streamers (up to twelve winches can be mounted on a seismic prospection boat). Furthermore, the logistics and means for storage of the paint are added to the other equipment present on the boat, which is already of a substantial number.